This invention relates to an iron-carbon composite containing an iron compound such as iron carbide or iron, and to a carbonaceous material comprising said iron-carbon composites.
Carbon nanotubes are a hollow carbon substance in which a graphite sheet (that is, a graphene sheet or carbon atomic plane with a graphite structure) is rolled into a tubular shape. The diameters of these tubes are on the nanometer scale, and the walls are of graphitic structure. These carbon nanotubes were discovered in 1991 by Dr. Sumio Iijima. Carbon nanotubes in which the wall structure consists of a single graphite sheet closed in a tubular shape are called single-walled carbon nanotubes, while those consisting of a plurality of graphite sheets each closed into a tubular shape and nested one within the other are called nested multi-walled carbon nanotubes.
Tubes that are similar to, but different in carbon wall structure from, the nested multi-walled carbon nanotubes, have been reported, in which the graphite wall structure is in a scroll form.
In an effort to improve the electrical characteristics in the field of electrical conductors and the like and the magnetic characteristics, there have been attempts in recent years to encapsulate a metal within the internal spaces defined by the tube walls of these carbon nanotubes (hereinafter sometimes referred to as xe2x80x9cCNTsxe2x80x9d) and so forth.
For example, Japanese Patent No. 2,546,114 discloses a foreign substance-containing carbon nanotube in which a substance other than carbon, such as a metal, is encapsulated in the cavity at the center of a nested multi-walled carbon nanotube. This foreign substance-containing carbon nanotube is prepared by vapor depositing a substance other than carbon at the end of a nested multi-walled carbon nanotube closed by a cap, either during or after the removal of the cap, and introducing the substance by thermal diffusion into the cavity located at the center of the carbon nanotube from the end of the carbon nanotube.
Japanese Unexamined Patent Publication No. 1997-142819 discloses carbon tubes each composed of a carbon nanotube having a diameter of 10 nm to 1 xcexcm and a length of 1 to 100 xcexcm and a foreign substance contained in the carbon nanotube. These foreign substance-containing carbon nanotubes are prepared using an inorganic substance having substantially straight channels as a template, either by coating the channel walls with an organic substance and carbonizing the organic substance by heating, or by subjecting a gaseous hydrocarbon to vapor phase carbonization inside the channels so as to deposit a thin film of carbon, thereby forming carbon tubes, and then bringing a solution or a melt of said foreign substance into contact with the tubes to insert the foreign substance into the cavities of the carbon nanotubes (and removing the inorganic substance by dissolving it before or after the insertion).
Japanese Unexamined Patent Publication No.2000-204471 discloses minute metal wires each composed of a wire material having a diameter of 1 to 100 nm and having a major axis length to diameter ratio of at least 50, and more particularly discloses a minute metal wire covered with a tube made of carbon. This minute metal wire covered with a carbon tube is prepared by substantially the same process as that disclosed in the above-mentioned Japanese Unexamined Patent Publication No. 1997-142819. The process comprises the first step of forming, in an inorganic substance having substantially linear channels, tubular carbon on the channel walls, and the second step of depositing metal in the interior of this tubular carbon.
However, the above-mentioned prior art processes require at least two steps of first forming a carbon tube and then inserting a foreign substance, so that the processes are complicated in terms of management and control of the steps, and entails low productivity. Furthermore, the preparation processes disclosed in the above-mentioned Japanese Unexamined Patent Publications Nos. 1997-142819 and 2000-204471 require a step of removing the inorganic substance used as a template by dissolving it.
Also, so far no process has been developed with which a composite comprising a metal, and particularly iron or an iron compound, contained in the internal space defined by the carbon wall of a tubular carbon material, such as carbon nanotube, can be obtained in amounts over the milligram scale. Consequently, practical research has not been done on such carbon-metal composites in which a metal or the like is contained in the internal space of a tubular carbon material.
The primary object of the present invention is to provide a composite in which iron or an iron compound is contained within a considerable portion of the internal space of a carbon tube, a carbonaceous material containing such composites, and processes for preparing the same.
The inventors conducted research in view of the status of the prior art as outlined above, and consequently discovered the following.
1) A carbon material comprising iron-carbon composites each composed of a carbon tube and iron or iron carbide contained in the internal space of the tube can be prepared in a single step by (1) heating an iron halide to 600 to 900xc2x0 C. in a reaction furnace in which the pressure therein has been adjusted to 10xe2x88x925 Pa to 200 kPa in an inert gas atmosphere and the oxygen concentration in the reaction furnace has been adjusted such that the ratio B/A is between 1xc3x9710xe2x88x9210 and 1xc3x9710xe2x88x921 wherein A is the reaction furnace volume (liters) and B is the amount of oxygen (Ncc), and (2) introducing an inert gas into the reaction furnace, and at a pressure of between 10xe2x88x925 Pa and 200 kPa, introducing thereinto a pyrolyzable carbon source and performing a heat treatment at 600 to 900xc2x0 C.
2) Tubes composed of carbon, obtained by controlling the cooling rate to a specified range in the cooling step after the above-mentioned step (2), are carbon tubes composed of a group of graphite sheets, which seem to be made up of a plurality of (usually many) flake-like graphite sheets formed into a patchwork or papier-mxc3xa2chxc3xa9-like structure. In this specification, these carbon tubes will be referred to as xe2x80x9cnanoflake carbon tubesxe2x80x9d. These nanoflake carbon tubes are tubular carbon materials that are completely different in structure from single-walled carbon nanotubes in which a single graphite sheet is closed into a cylindrical form, or from concentric cylindrical or nested multi-walled carbon nanotubes in which a plurality of graphite sheets are each closed into a cylindrical form.
3) As for the internal spaces within the nanoflake carbon tubes (that is, the space defined by the walls of the nanoflake carbon tubes), a considerable portion, particularly 10 to 90%, of the space is filled with iron or iron carbide, forming an iron-carbon composite.
4) The tube composed of carbon, obtained by performing a heat treatment in an inert gas and cooling at a specific cooling rate following the above-mentioned step (2), is a multi-walled carbon nanotube of a nested structure. A considerable portion of the internal space of the multi-walled carbon nanotube, particularly 10 to 90% of this space, is filled with iron or iron carbide, forming an iron-carbon composite.
5) The above composites, each composed of a carbon tube selected from the group consisting of nanoflake carbon tube and nested multi-walled carbon nanotube and a metal (particularly iron or iron carbide) contained in the carbon tube, are useful as an electron emitting material that can emit electrons at a high current density under a low electrical field.
The present invention was achieved by carrying out further investigation on the basis of these findings, and provides the following iron-carbon composite and its preparation process.
Item 1 An iron-carbon composite composed of (a) a carbon tube selected from the group consisting of nanoflake carbon tubes and nested multi-walled carbon nanotubes and (b) iron carbide or iron, wherein 10 to 90% of the internal space of the carbon tube is filled with the iron carbide or iron.
Item 2 The iron-carbon composite according to Item 1 above, which has a straight shape, an outside diameter of 1 to 100 nm, and a carbon wall thickness of 49 nm or less, the carbon wall thickness being substantially uniform over the entire length, and has an aspect ratio L/D of 5 to 10,000 wherein L is the length and D is the outside diameter.
Item 3 The iron-carbon composite according to Item 1 or 2 above, wherein the mean spacing between the hexagonal carbon layers (d002) is 0.34 nm or less, as determined by applying X-ray diffractometry to the wall of the carbon tube that makes up the iron-carbon composite.
Item 4 The iron-carbon composite according to any one of Items 1 to 3 above, wherein the carbon tube that makes up the iron-carbon composite is a nanoflake carbon tube.
Item 5 The iron-carbon composite according to any one of Items 1 to 3 above, wherein the carbon tube that makes up the iron-carbon composite is a nested multi-walled carbon nanotube.
Item 6 A carbonaceous material comprising iron-carbon composites composed of (a) carbon tubes selected from the group consisting of nanoflake carbon tubes and nested multi-walled carbon nanotubes and (b) iron carbide or iron, wherein 10 to 90% of the internal space each of the carbon tubes is filled with the iron carbide or iron.
Item 7 The carbonaceous material according to Item 6 above, wherein the ratio R of Ia/Ib is 0.35 to 5 as determined by powder X-ray diffractometry in which the carbonaceous material is irradiated with CuKxcex1 X-ray over an irradiation area of at least 25 mm2 per mg of the carbonaceous material, wherein Ia is the integrated intensity of the peak which has the strongest integrated intensity among the peaks appearing in the range of 40xc2x0 less than 2xcex8 less than 50xc2x0 assigned to the iron or iron carbide contained in the carbon tubes, and Ib is the integrated intensity of the peak appearing in the range of 26xc2x0 less than 2xcex8 less than 27xc2x0 assigned to the mean spacing between the hexagonal carbon layers (d002) of the carbon tubes.
Item 8 The carbonaceous material according to Item 6 or 7 above, wherein the iron-carbon composites have straight shapes, outside diameters of 1 to 100 nm, carbon wall thicknesses of 49 nm or less, the carbon wall thicknesses being substantially uniform over the entire lengths, and also have aspect ratios L/D in the range of 5 to 10,000 where L is the length and D is the outside diameter.
Item 9 The carbonaceous material according to any one of Items 6 to 8 above, wherein the mean spacing between the hexagonal carbon layers (d002) is 0.34 nm or less, as determined by applying X-ray diffractometry to the walls of the carbon tubes that make up the iron-carbon composites.
Item 10 The carbonaceous material according to any one of Items 6 to 9 above, wherein the carbon tubes that make up the iron-carbon composites are nanoflake carbon tubes.
Item 11 The carbonaceous material according to any one of Items 6 to 9 above, wherein the carbon tubes that make up the iron-carbon composites are nested multi-walled carbon nanotubes.
Item 12 A process for producing a carbonaceous material comprising iron-carbon composites composed of (a) carbon tubes selected from the group consisting of nanoflake carbon tubes and nested multi-walled carbon nanotubes and (b) iron carbide or iron, wherein 10 to 90% of the internal space of each carbon tube is filled with the iron carbide or iron, said process comprising the steps of:
(1) heating an iron halide to a temperature of 600 to 900xc2x0 C. in a reaction furnace in which the pressure has been adjusted to 10xe2x88x925 Pa to 200 kPa in an inert gas atmosphere and the oxygen concentration in the reaction furnace has been adjusted such that the ratio B/A is 1xc3x9710xe2x88x9210 to 1xc3x9710xe2x88x921 wherein A is the reaction furnace volume (liters) and B is the oxygen quantity (Ncc); and
(2) introducing an inert gas to the reaction furnace, and at a pressure of between 10xe2x88x925 Pa and 200 kPa, introducing a pyrolyzable carbon source and performing a heat treatment at 600 to 900xc2x0 C.
Item 13 The process according to Item 12 above, which comprises, after the heat treatment in step (2), cooling the reaction furnace to 500xc2x0 C. at a rate of 50 to 2000xc2x0 C./h to thereby produce a carbonaceous material comprising iron-carbon composites composed of nanoflake carbon tubes and iron carbide or iron that fills 10 to 90% of the internal space of each tube.
Item 14 The process according to Item 12 above, which, after the heat treatment step in step (2), comprises the steps of:
(3) replacing the atmosphere inside the reaction furnace with an inert gas while the temperature in step (2) is maintained;
(4) elevating the temperature in the reaction furnace, the atmosphere of which has been replaced with the inert gas, to 950 to 1500xc2x0 C.;
(5) maintaining the final temperature at the end of the temperature elevation until nested multi-walled carbon nanotubes are produced; and
(6) cooling the temperature in the reaction furnace at a rate of 50xc2x0 C./h or lower, to thereby produce a carbonaceous material comprising iron-carbon composites composed of nested multi-walled carbon nanotubes and iron carbide or iron that fills 10 to 90% of the spaces inside the tubes.
Item 15 The process according to Item 12 above, wherein the heat treatment in step (2) is performed in the presence of an organic iron complex.
Item 16 The process according to Item 15 above, wherein the organic iron complex is ferrocene or an iron carbonyl complex.
Item 17 The process according to any one of Items 12 to 16 above, wherein the iron halide is an iron chloride.
Item 18 The process according to Item 17 above, wherein the iron chloride is at least one member selected from the group consisting of FeCl2, FeCl3, FeCl2.4H2O and FeCl3.6H2O.
Item 19 The process according to any one of Items 12 to 18 above, wherein the pyrolyzable carbon source is at least one member selected from the group consisting of aromatic hydrocarbons having 6 to 12 carbon atoms, saturated aliphatic hydrocarbons having 1 to 10 carbon atoms, and unsaturated aliphatic hydrocarbons having 2 to 5 carbon atoms.